In image-forming apparatus such as electrophotographic apparatus (e.g., copiers and printers), examples of causes of image blurring and the like include working allowances and assembly allowances of various components of the image-forming apparatus, vibration of the image-forming apparatus, and rotation unevenness of photoreceptor drums attributable to gear transmissions.
The present inventor directed attention to a photoreceptor drum itself, which is a major component of an image-forming apparatus, in order to obtain clear images.
Namely, a photoreceptor drum is constituted of a drum main body having a photosensitive layer formed thereon and a flange member to be attached to one end of the drum main body. The photoreceptor drum is disposed in an image-forming apparatus so that it rotates on a central axis which connects the center of a shaft support part formed at the other end of the drum main body to the center of a shaft support part formed in or on the flange member. Alternatively, a photoreceptor drum is constituted of a drum main body and flange members to be attached respectively to both ends of the drum main body, and this photoreceptor drum is disposed in an image-forming apparatus so that it rotates on a central axis which connects the centers of shaft support parts of the respective flange members at both ends of the drum main body.
When the photoreceptor drum has deflection, i.e., when the photoreceptor drum is bent (the photoreceptor drum itself is in a bent state) or has axis position shifting (the state in which the center of the outer circumference of the photoreceptor drum differs from the center of rotation), then the deflection attributable to the bending or axis position shifting, during the formation and transfer of electrostatic latent images, causes image shifting from the positions where images are to be formed.
Illustratively stated, in an apparatus in which laser scanning with a polygonal mirror is conducted, such as a laser beam printer, laser beam incidence during the formation of an electrostatic latent image becomes more oblique as the image-forming position approaches an end of the photoreceptor drum. Because of this, in case where the photoreceptor drum is bent or has axis position shifting, main-scanning-direction shifting occurs in which the positions to which the laser beam reaches shift in the drum axis directions.
Furthermore, when the photoreceptor drum is bent or has axis position shifting, it has unevenness in the distance from the center of rotation of the photoreceptor drum to the surface of the photoreceptor drum, i.e., in radius of rotation. Because of this, in areas which have a small radius of rotation, the photoreceptor drum surface has a reduced rate of movement relative to the exposure system, resulting in a contracted electrostatic image. In areas which have a large radius of rotation, the photoreceptor drum surface has an increased rate of movement relative to the exposure system, resulting in an elongated electrostatic image. Namely, side-scanning-direction shifting occurs.
As a result, the image printed is distorted. Especially in the case of a color copier of the type called tandem, in which photoreceptor drums arranged in parallel are used, those troubles markedly appear as positional shifting and color shifting because the photoreceptor drums are separately used for respective colors.
In an apparatus employing a light-emitting diode as an illuminator, as another kind of apparatus, rotation with deflection is apt to cause image blurring because of the short focal distance and this problem is serious.
As long as the photoreceptor drum has been assembled as designed and has no deflection, problems such as those described above do not arise. However, it is virtually almost impossible to obtain a photoreceptor drum completely free from deflection.
When attention is directed to photoreceptor drums only, there is no quantitative standard as to how high the degree of deflection of a photoreceptor drum is when clear images are not obtained, or as to how narrow the range of working allowances is when clear images can be obtained.
As a result, various expedients have hitherto been taken for obtaining clear images from standpoints not concerning photoreceptor drums themselves. For example, reduction in assembly allowances and inhibition of apparatus vibration are attempted on the assumption that some degree of deflection is unavoidable, or reduction in the rotation unevenness of a photoreceptor drum is attempted (e.g., JP-A-2000-330448).
However, these expedients have come to have limitations with the recent trend toward resolution increase and color image formation in image-forming apparatus. Some further improvement has been desired.
The invention has been achieved under the circumstances described above. An object of the invention is to provide a photoreceptor drum exceedingly advantageous for obtaining clear images free from positional image shifting and image blurring.
Another object of the invention is to provide a method and apparatus by which a photoreceptor drum capable of giving clear images can be easily assembled without fail.
A still other object of the invention is to provide an image-forming apparatus capable of giving clear images.